Potassium channels have been found in the apical and basolateral membranes of renal proximal straight tubules, but the fraction of apical membrane conductance due to or associated with potassium has not been reported. Therefore, the transepithelial conductance of rabbit pars recta was measured and the portion of conductance which was inhibited by barium determined. The mean control transepithelial voltage and conductance were -5.6 +/- 0.7 mV and 106 +/- 10 mS/cm2, respectively. The addition of barium to the luminal perfusing solution decreased transepithelial conductance to 93 +/- 9 mS/cm2. This decrease of 13 +/- 2% from control (p < 0.001, n = 20) cannot be explained solely by inhibition of an apical potassium conductance. It is proposed that the apical potassium channels found in the pars recta facilitate transepithelial conductive transport by sodium and other ions.